1. Field of the Invention
This invention relates to an electronic amplification system for musical instruments and, more particularly, relates to a variable power amplifier adapted for receiving preprocessed audio frequency electrical signals generated by a musical instrument and for providing selectively amplified output signals to an amplifier output terminal for driving an output device, such as a loudspeaker.
2. Description of the Prior Art
It is known in the art to utilize performance amplifiers in connection with musical instruments, such as an electrical guitar, wherein the output thereof has its amplitude, frequency and harmonic distortion modified by the amplifier prior to application of the modified electrical signals to an output device. In the state-of-the-art musical instrument performance amplifiers, the amplifiers traditionally use solid-state devices, such as NPN transistors or PNP transistors, field effect transistors, and the like. One such musical instrument performance amplifier is disclosed in U.S. Pat. No. 4,345,502, which utilizes solid-state devices in the amplifier. In the musical instrument performance amplifier of U.S. Pat. No. 4,345,502, a pair of high-impedance power field effect transistors are connected to an output transformer to provide a relatively low push-pull drain load that effectively presents a high-impedance constant current source to an amplifier. The amplifier, having such high-impedance constant current source output, amplifies and sustains instrument sound, provides large amounts of power to the speaker over a wide range of speaker impedances and enables the production of high-power harmonics through the speaker. In such amplifiers, the use of high-impedance field effect transistors having a very high gain allows, in turn, the use of an output section having no feedback to the output transistors.
Other known musical instrument performance amplifiers utilize vacuum tubes as the active elements. One such tube-type amplifier is offered for sale and sold under the trademark TUSC, and one of the series is identified as the "Prestige Series" programmable tube-type amplifiers. The TUSC tube-type amplifier utilizes a single channel which is adapted to be programmable to have two different operating characteristics such that it simulates a two-channel system. The TUSC tube-type amplifier includes a programmable memory which contains memory means for providing a first preprogrammed set of settings which can utilize the channel as a programmable A channel and wherein the settings on the single amplifier can be changed to a second preprogrammed setting forming a second channel, or program B channel. The programmable memory feature permits programmable overdrive and parametric equalization settings which permit the user to obtain a wider range of tonal possibilities.
U.S. Pat. No. 3,663,735 discloses an arrangement employing two channels and ganged switching devices at the output of each of a pair of channels. One channel includes a means for producing a particular amount of distortion and the other produces a clean, undistorted version of the signal. The switching device in U.S. Pat. No. 3,663,735 employs ganged switching devices at the output of each of a pair of channels under control of a moving foot switch for choosing one channel or the other channel. This arrangement requires relatively complex circuitry and simultaneous control of several switches.
U.S. Pat. No. 3,973,461 discloses a distortion control circuit which includes an input terminal which is fed to a two-terminal system having a distortion channel which includes a distortion circuit and a second channel designated as a clean channel wherein the output of both the distortion channel and the clean channel output signals are concurrently applied across a variable resistor. In this arrangement, the distortion channel provides an output that is in phase with the input thereto, whereby the signals in the two channels will not cancel regardless of the position of the adjustable resistor. In this arrangement, under control of an off-on signal, the distortion channel can be shut off, permittng the signals of the clean channel only to be applied across the variable resistor. When the switch position is changed, the distortion channel is then switched into the circuit and in parallel circuit relationship to the clean channel. This results in an output signal being produced which is a combination of the signals produced by the distortion channel and transmitted by the clean channel across the variable resistor. In the distortion control circuit disclosed in U.S. Pat. No. 3,973,461, either one or both channels are operative to produce either a clean signal or a clean signal having distortion.
U.S. Pat. No. 4,211,893 discloses a dual-mode musical instrument wherein a single-channel amplifying apparatus includes a final output stage which can be switched into the circuit or switched out of the circuit, depending on the type of musical signal desired by the operator. Specifically, the last output stage, which is a triode, is operated such that large amplitude input signals having both positive and negative excursions drive the tube to operate in the nonlinear portions of its characteristics, resulting in an output of distorted signals having both even and odd harmonics. Also, U.S. Pat. No. 4,211,893 discloses that some preceding stages, which are essentially stages of amplification, can either be dual-triode electronic tubes or field effect transistors directly interchangeable with the vacuum tubes. Also, the overdrive saturation principles in the dual-mode musical instrument amplifier can likewise be obtained by any combination of vacuum tubes, field effect transistors or bipolar transistors.
U.S. Pat. No. 2,315,248 discloses an amplification system which is adapted to provide a pseudo-extension of the frequency band of the output signals generated by the amplifier. U.S. Pat. No. 2,315,248 discloses the concept of utilizing two channels, having two amplifiers and two input microphones which are responsive to selected frequency ranges, and the outputs of each of the two channels are then combined by means of a mixing tube which produces an output signal which is a composite of the signals passed by the two separate channels. The amplifier of U.S. Pat. No. 2,315,248 modifies and distorts the tonal quality of audio frequency signals generated by a musical instrument by overdriving the electronic discharge device of one of the stages of the amplifier and providing a nonlinear output.
The operation of triode tubes, pentode tubes, and the like, in amplifiers adapted for Class A, Class AB, Class B and Class C operation are disclosed in a number of references, one of which is the Radio Amateur's Handbook by the Headquarters Staff of the American Radio Relay League of Newington, Conn. 06111, U.S.A., as set forth in the 45th Edition dated 1968. The Radio Amateur's Handbook includes a detailed discussion of the method for biasing tubes for linear and nonlinear operation, the operation of various types of solid-state devices including field effect transistors and insulated gate field effect transistors. In addition, circuit diagrams, base tube diagrams and a description of miniature receiving tube characteristics are likewise set forth in the Radio Amateur's Handbook.
It is also known in the art to utilize a variable current source in the driving section of the amplifier and to limit and/or control drive capability using a resistor bank wherein the absorbed electrical power is a function of the current which limits the output thereof. One known amplifier which uses a variable current source in the driver is a Model D 180 amplifier offered for sale and sold by Mesa Boggie.
Known methods utilized in vacuum tube amplifiers for varying the output power of the amplifier include use of passive elements, such as the amplifier disclosed in U.S. Pat. No. 4,143,235.
U.S. Pat. No. 4,143,245 discloses a device for maintaining a constant distortion output from a musical instrument amplifier over a volume range of the musical system. The amplifier is set at a predesired volume setting and volume control is provided by a potentiometer control interposed between the output of the amplifier which may, for example, be a guitar amplifier, and the loudspeaker. The potentiometer is connected so that one end is in series with the speaker providing the primary means for controlling volume while the other end is in parallel with the speaker. Because the load that the amplifier sees is a function of both the speaker impedance and the potentiometer setting, there is a calibration scale associated with the control dial of the potentiometer so that the amplifier output impedance can be matched to the impedance of the speaker being used.